An uninterruptible power supply unit is a power supply unit that supplies power to a load device from a secondary battery which is charged in advance to allow the load device to continue its operation when power supply to the load device from an external power supply is discontinued due to a power breakdown, etc. It is general that the secondary battery for the uninterruptible power supply unit is charged with power of an external power supply during normal time. As an example of the secondary battery used for the uninterruptible power supply unit, for example, an alkaline secondary battery such as a nickel-hydrogen secondary battery is publicly known.
An alkaline secondary battery, by its nature, needs to be charged with a voltage higher than its rated voltage. However, in general, in an uninterruptible power supply unit, the rated voltage of the alkaline secondary battery is a voltage same as the voltage of the external power supply. For that reason, it is not possible to charge the alkaline secondary battery to a fully charged state with the power of the external power supply as it is.
Under these circumstances, an uninterruptible power supply unit equipped with an auxiliary power supply (DC/DC converter) for boosting the voltage of the external power supply is publicly known. To be more specific, when the alkaline secondary battery is charged, the voltage of the external power supply is boosted by an auxiliary power supply and the alkaline secondary battery is charged with the boosted voltage. As a result of this, even if the alkaline secondary battery has a rated voltage which is same as the voltage of the external power supply, it is possible to charge the alkaline secondary battery to a fully charged state with a voltage higher than the rated voltage. Moreover, as an example of technique which utilizes an auxiliary power supply, though which is not an uninterruptible power supply unit, a motor control unit in which the amount of power which can be supplied to an electric motor is increased by the auxiliary power supply when power demand of the electric motor temporarily increases is publicly known (for example, see Patent Document 1).
However, the conventional art which boosts the voltage of the external power supply with an auxiliary power source, and charges the alkaline secondary battery with the boosted voltage has a problem in that significant power loss occurs during charging. Moreover, in such the conventional art, heat generation of the auxiliary power supply caused by the power loss may lead to a problem in terms of the reliability of the uninterruptible power supply unit. To address such problem, for example, there is publicly known a charging control unit for performing split charging control, in which when a secondary battery including a plurality of unit cells connected in series is charged, some of the plurality of unit cells are selectively connected to a power converter (for example, see Patent Document 2). Since the charging control unit for performing such split charging control selectively charges a plurality of unit cells constituting the secondary battery, it is possible to use a smaller-scale power converter. Since, thereby the charging control unit for performing split charging control can decrease power loss in the power converter, it can reduce the heat generation of the power converter.